(1) Field of the Invention
The present invention relates to telephone switching systems and more particularly to a clock selection circuit for use in a telephone switching system having a plurality of clock circuits.
(2) Description of the Prior Art
Telephone switching systems have been equipped with redundant clock circuits to prevent interruptions in service due to failure of a clock circuit. These telephone systems typically include a pair of clock circuits arranged in an active-standby manner. Such an arrangement is based on the assumption that only one fault can exist at one time and therefore two clock circuits are deemed sufficient. Only minimal logic circuitry was needed to control selection of active and standby clock circuits since it need only detect failure of the active clock circuit and then switch to the standby clock circuit. However such systems are subject to interruptions in service should there be a failure in the standby clock circuit which is forced on line upon detection of a failure in the active clock circuit.
In order to provide increased reliability additional clock circuits are required. More sophisticated clock selection circuitry must also be provided to insure that only a properly operating standby clock circuit is switched on line upon detection of a failure in the active clock circuit.
Accordingly, it is the object of the present invention to provide a highly reliable clock selection circuit capable of selecting a properly functioning clock circuit from a plurality of available clock circuits, upon detection of a failure in the on-line clock circuit.